Too Hot for Large Animals?

The old adage that bigger is better could be
about to go out of fashion. Ecologists say
climate change will shrink species.

But don’t look out for hot shrinking animals just
yet – the effects are likely not to be seen for
many more years. Yet Kaustuv Roy, a biologist at
the University of California in San Diego,
believes we need to think now about how we are
going to preserve large species.

“Our collective actions are negatively affecting
body sizes of many living species,” says Roy. It
is well-known that humans tend to hunt or fish
larger animals, creating a selective pressure
that favours the smaller ones that can reproduce
while they are still small. Several species of
cod are smaller as a result of pressures of the
fishing industry.

The degradation of natural environments around
the world is having the same effect by limiting
the amount of food available to animals, says Roy
– meaning smaller animals that need less food
have a head start.

But Roy believes another factor threatens the
world’s most impressive animals. “Global warming
may reinforce this trend towards smaller sizes
through the temperature-size rule,” he says.

Size rules

The temperature-size rule, also known as
Bergmann’s rule, says that species size increases
with latitude: they tend to be smaller in the
tropics, and larger closer to the poles.
Bergmann’s rule is debated, but one explanation
for it is that larger animals have a lower
surface-area-to-volume ratio, allowing them to
retain more heat and fare better in cooler climes.

Conversely, smaller species radiate their heat
more easily and so are better adapted to living
in warm temperatures. There is also some
experimental evidence that rearing animals in
higher temperatures generally results in smaller
individuals.

Roy has shown that the average body size of tiny
ocean shrimps, known as ostracodes, gradually
became bigger several million years ago. As the
world cooled by 12°C during the Cenozoic era,
fossil ostracodes got about 30 microns bigger for
every degree of cooling (PNAS, DOI:
10.1073/pnas.0510550103). Although he does not
yet have the data, he now says we should expect
the opposite consequence from human-induced
global warming.

“In effect, our actions have set up a grand
selection experiment where bigger is no longer
better,” says Roy, who has also shown that
species evolve faster in cooler temperatures.

Looking for the small

“It makes sense to be bigger when it’s colder,”
says Wendy Foden, a biologist at the World
Conservation Union who is studying the effects of
climate change on species. “As the world gets
warmer, the converse will happen, species will
shrink.”

Andy Purvis of Imperial College’s Centre for
Population Biology in the UK agrees. “There are
bound to be exceptions – species that find energy
more easy to come by in a warmer climate – but
generally I would expect species to shrink
because they’ll be coping with sudden changes in
their environment and dying younger.”

In 2005, Purvis was part of a study showing that
species that are threatened with extinction are
on average one order of magnitude larger than
those that are not (Science, DOI:
10.1126/science.1116030).

To Foden and Purvis’ knowledge, ecologists have
yet to identify a species that has shrunk as a
result of global warming – though many have
shrunk because of fishing, hunting, and the
degradation of their habitats. Because the
shrinking is an evolutionary response, it will
take a long time for it to be noticeable.

Foden says the most likely place to spot it would
be in an environment that has already experienced
significant warming, and among species with short
generation times. So it might be time to start
scrutinising animals in the Arctic.